The present invention is directed to methods and apparatus for forming pellets of thermoplastic materials such as polystyrene, polyethylene, polyvinylchloride, and polypropylene, among others. The pelletizing of such materials with incorporated additives is common because the pellets may be conveniently measured, packed, shipped, stored and used by the end user.
While many pelletizing systems find the thermoplastic material, in a flowable state, being expressed under pressure through orifices in a stationary die, as by a screw extruder, the useful efficiency (hydraulic power/total power supplied) of such systems is quite low.
To overcome the foregoing, and other problems associated with the stationary die type extrusion, centrifugal extruders have been developed in which thermoplastic material in a flowable state is fed into the interior of a hollow rotor to be centrifugally expressed through orifices in the rim of the rotor (see, for example, U.S. Pat. Nos. 3,358,323; 3,424,832; and 3,483,281).
The present invention is especially directed to improvements in pelletizers of the centrifugal type wherein the centrifugal head developed in the rotary polymer melt extruder is used to generate the pressure required for extrusion and the extruder is self-emptying. In such pelletizers there is no need to further heat the polymer because of insufficient pressure development (advantageous with thermally sensitive polymers) and an eddy current heating system can be efficiently and economically used to compensate for heat lost by the spinning rotor to the surroundings.
A prime object of the present invention is to provide certain novel improvements in methods of centrifugally pelletizing, and centrifugal pelletizing systems, which greatly enhance the practicality of such systems and will hereinafter be described in detail and claimed.